(1) Field of the Invention
The present invention relates to methods of connecting a flexible upper riser part to a lower riser part of a hybrid riser via an upper riser assembly.
The basics of riser drilling are described and illustrated in “D/V Chikyu, Riser Operations and the Future of Scientific Ocean Drilling” published in Oceanography, Vol. 19, No. 4, December 2006. Here it is described that riser drilling involves several steps that are accomplished in a variety of ways depending on specific technological packages, water depths and bottom conditions. First, a wide diameter pilot hole is drilled and cased, then a specialized wellhead is provided to anchor a pressure sensor and finally a shut-off valve assembly, called a blow-out preventer (BOP), is installed. The BOP itself may be lowered to the wellhead and attached using a remotely operated vehicle (ROV). Lowering of the BOP is accomplished by successively attaching sections of riser pipe to the top of the BOP and lowering them to the sea floor. Riser pipe itself comprises wide diameter high-strength pipe with external conduits for cables and connectors to allow control and monitoring of the BOP. Once the BOP is installed at the wellhead and linked to the drilling vessel via the riser pipes, drilling and coring, as well as any downhole logging, measurement operations or sampling operations can begin.
More recently, the free-standing hybrid riser (FSHR) system has been developed as an attractive solution for deep water operations due to its much reduced dynamic response as a result of significant motion decoupling between the vessel and the riser and due to the same vessel interface loads that it presents when compared with steel catenary risers (SCRs) or flexible pipe solutions.
A deep water riser assembly of this type is described in U.S. Pat. No. 5,676,209. The assembly includes a lower blow-out preventer (BOP) stack positioned adjacent and anchored to the bottom of the ocean and an upper BOP stack attached to the riser at water level, but just far enough below the water surface to be unaffected by surface currents. The upper BOP stack has shear rams above the pipe rams to sever the section of the drill pipe above the shear rams and allow the upper section of the drill pipe between the shear rams and the drill ship to be retrieved followed by the section of riser above the upper BOP stack. This frees the drill ship to move as required in order better to weather a surface storm. A floatation module is attached to the riser below the upper BOP stack and exerts an upward force that holds the riser below the upper BOP stack free standing and in tension. Means are provided to reconnect the upper section of the riser to the upper BOP stack after the storm has passed.
(2) Description of Related Art
Another related riser design is described in US Publication No. US 2008/0302535 A1. In this document, a multi-component system for subsea intervention is described. The system comprises a lower riser component which is held vertical by a buoyancy element and an upper riser system. The upper riser system comprises a continuous enjoined conduit with sufficient flexibility to absorb the motion of the deployment vessel without adversely affecting the function of the intervention system.
In one known FSHR system, a single vertical steel pipe connected to a foundation pile at the sea bed. The system is tensioned using a nitrogen-filled buoyancy tank which is mechanically connected to the riser. In one variant, the riser pipe runs through the bore of the buoyancy tank which is located below the mean water level out of a wave and high current zone. At the top of the buoyancy tank a goose neck assembly is provided, to which a flexible juniper on the riser is attached to link the riser to the vessel, thus essentially decoupling the free-standing riser from the vessel motions.
In systems where a central pipe runs through the centre of the buoyancy tank, this acts as the main structural element in the buoyancy tank. Internal bulk heads are used to divide the tank into sub-compartments. The riser pipe is attached to a load shoulder on the top of the buoyancy tank and thus the upthrust generated by the buoyancy tank is transmitted directly to the pipe to provide tension in the riser string.
The goose neck assembly provides fluid take off from the free-standing riser to the flexible jumper. It comprises an induction bend and is structurally braced back to a goose neck support spool at the base of the assembly to react the loads generated by the flexible jumper. Positioning of the goose neck at the top of the buoyancy tank allows for independent installation of vertical riser and flexible jumper. The flexible pipe installation vessel can install the flexible jumper at a convenient time. This minimizes the risk of damage to the flexible jumper during installation as the procedure is similar to that of a shallow water flexible riser with the first end at the top of the buoyancy tank.
However, the position of the goose neck relative to the buoyancy tank can be varied. In an alternative design, the goose neck is positioned below the buoyancy tank and the vertical riser is tensioned by the tank via a flexible linkage. This arrangement simplifies the interface between the buoyancy tank and the vertical riser and allows preassembly of the flexible jumper to the goose neck before deployment of the vertical riser. However, in the known systems, in the event of a flexible jumper replacement or repair, an elaborate jumper disconnection system has to be employed below the buoyancy tank.
Reference is also made to U.S. Pat. No. 3,717,002 A, which discloses, with reference to FIG. 15 thereof, a method and apparatus for lowering from a platform a constructed, vertical, upper riser into mating and interconnecting engagement with an underwater connecting end at the upper end of an underwater pipeline. A crane used for lowering the constructed pipeline on a first cable onto the pipeline connecting end also supports a coupling cable which passes down through the constructed riser and is connected, by means of a hook on the end of the cable, to a guide coupling assembly positioned on the underwater connecting end of the underwater pipeline. The vertical riser would appear to be made up of individual interconnected rigid riser sections.